The invention is particularly applicable to the direct viewing screens market: from cell phone screens to large television screens. It is more particularly applicable to displays for which pixels are controlled analogically. In a corresponding addressing method, the video to be displayed on a pixel is controlled by an analogue voltage level output by a digital analogue converter using the received digital video signal. To display a given grey level, the active element of a pixel is activated for one row period to transfer a corresponding analogue voltage level onto the pixel capacitance. The liquid crystal is then oriented in a direction that depends on the applied analogue value. Input light bias passing through this liquid crystal is then modified and analysed by a polariser.
The display performance depends particularly on the brightness that depends on the pixel illumination time. This illumination time depends on the addressing time necessary to transfer analogue voltage levels onto each pixel in a row of the matrix, and the liquid crystal stabilization time that depends on the previous analogue voltage level and the current analogue voltage level.
These constraints are accentuated in the case of a display addressed in sequential colour mode. Remember that in a display, the pixels are organised in rows and columns of a matrix, each pixel being arranged at the intersection of a row and a column. The rows of cells are addressed sequentially. The cells in a row are addressed simultaneously and receive new analogue data through columns during the row time (in other words the time during which the associated row is selected). An addressing phase of a display normally includes a write step during which rows are selected sequentially, and the active elements of each selected row are activated to receive and transfer the analogue voltage level onto the associated pixel capacitance, with a stabilization time corresponding to the changeover time necessary so that all pixels are switched; and an illumination time during which the panel is illuminated, the light is modulated by the display and the corresponding image is recovered. In the case of a sequential colour mode, these steps are performed once for each primary colour within a particular video frame. There are generally three coloured sub-frames per frame. Thus, within the time of one frame, all pixels in the matrix have to be addressed at least three times to display the video information corresponding to each primary colour. It may be necessary to have two or more coloured sub-frames per primary colour in a video frame, to avoid perceiving the different coloured frames.
There are still some problems that arise with these displays, particularly due to the liquid crystal response time that depends on the analogue voltage level memorized during the previous sub-frame and the analogue voltage level to be charged during the new sub-frame. Taking the example of a TN (Twisted Nematic) type liquid crystal display, the changeover time for the liquid crystal to change from one grey level to another is much greater than the changeover time to change from the black level to the white level, or from the white level to a grey level. The black level is the liquid crystal mode in which the potential difference between the pixel and the counter electrode is maximum. The white level is the liquid crystal mode in which the potential difference between the pixel and the counter electrode is minimum. A grey level is an intermediate level: light greys correspond to an applied potential difference similar to the value applied to obtain white, while dark greys correspond to a potential difference similar to that applied to obtain black. The changeover time to change from the white level to a light grey level may be one and a half times longer than the changeover time to change from the black level to this same light grey level. The changeover time from a light grey level to the black level is very short. In one example with TN type liquid crystals, the following changeover times were measured: 0.2 milliseconds to change from white to black; 1 millisecond to change from black to white; 3.25 milliseconds in the worst case measured between two grey levels.